This invention relates to a semiconductor laser and to a semiconductor light-emitting element. In particular, this invention relates to a semiconductor laser provided with a p-type semiconductor film and a p-type layer and also to a semiconductor light-emitting element provided with a p-type semiconductor film and a p-type layer.
In recent years, a III-V Group compound semiconductor, such as GaAs, InP and GaInAlP, can be grown, by a metal organic chemical vapor deposition (MOCVD), with better controllability and has been extensively utilized as a constituent component for a semiconductor laser and a light-emitting diode.
In the MOCVD method, Zn is generally used as a p-type dopant for the III-V Group compound semiconductor and, being used as a p-type dopant for the GaAs, exhibits a substantially good doping characteristic. If, however, Zn is used as a dopant for a p-type containing III-V Group compound semiconductor, such as InP and GaInAlP, the quantity of Zn that can be incorporated into the compound semiconductor is limited and it has been difficult to dope Zn in a desired amount. Further, the activation degree of Zn is low and the diffusion of it in a layer is fast, resulting in poor controllability.
The elements Be and Mg for example may be considered as a p-type dopant in place of Zn. The element Be exhibits a good characteristic as a p-type dopant in a molecular beam epitaxy (MBE) method.
However, the organic Be compound is strongly toxic and it is very difficult to use it as the dopant in the MOCVD method. On the other hand, a straight chain type organometallic compound of Mg, such as dimethylmagnesium and diethylmagnesium, is not toxic in nature, but very strong in its self-association and never occurs in single form. For this reason, the Mg-containing straight chain type organometallic compound is not suitable as a doping agent.
Recently, biscyclopentadienylmagnesium (Cp.sub.2 Mg) relatively high in vapor pressure has been used as a Mg-doping material. However, the material Cp.sub.2 Mg is deposited as a residual one in a crystal growing apparatus and exhibits a memory effect so that the doping control is very difficult. In spite of the fact that a three-orders-of-magnitude-greater concentration variation is required in a 0.1 .mu.m film thickness for a double heterostructure (DH) laser device, such a sharp concentration variation cannot be ensured at the present time. In order to enhance the vapor pressure, the methyl group-attached cyclopentane ring material "bismethylcyclopentadienyl magnesium" [(CH.sub.3)Cp.sub.2 Mg] is known as one example of a dopant and, even in this case, no sharp Mg-concentration variation is obtained at a doped-to-undoped interface.
In order to reduce a resistivity of a predetermined semiconductor layer, more amount of Mg has to be doped as a p-type dopant. In order to obtain a resistance value of, for example, about 0.5 .OMEGA..cm to 10 .OMEGA..cm, the Mg has to be doped at a concentration amount of about 5.times.10.sup.18 /cm.sup.3 to 5.times.10.sup.19 / cm.sup.3. In the case where more amount of Mg is so doped, the laser performance is lowered and, for the case of a semiconductor laser having an active layer as narrow as below 5 nm in thickness in particular, a greater adverse influence is exerted over an operating current.
In the case where the Mg doping amount is increased so as to lower the resistance of a p-type cladding layer, a semiconductor laser principally composed of a Mg-doped III-V Group semiconductor is lower in the characteristics, such as an efficiency of an emitting layer and, in the worst case, the operation is not possible.
In either case, if the Mg amount is increased in a semiconductor layer, the semiconductor laser becomes poor in its characteristics, thus failing to obtain adequate reliability.
By the way, a nitride compound semiconductor such as GaN has been attracting attention nowadays as a material for a blue light-emitting diode or for a semiconductor laser. For example, a blue light-emitting diode and a semiconductor laser have been realized by making use of the nitride compound semiconductor. This element is featured as being formed of a so-called double hetero-structure wherein a light-emitting layer is sandwiched by a pair of materials, one of which having a p-type conductivity and the other having an n-type conductivity. In this case, the energy band gaps of the p-type and n-type materials are larger than that of the light-emitting layer. It is required however in the formation of a p-type nitride compound semiconductor layer to undergo a step of electron ray irradiation or of thermal annealing after the p-type nitride compound semiconductor layer has been grown by way of an MOCVD as described for instance in Japanese Patent Unexamined Publication H/2-257679 or Japanese Patent Unexamined Publication H/5-183189. However, these steps may bring about a cause for generating a crystal defect such as a nitrogen void which is peculiar to a nitride compound semiconductor. Due to these reasons, even though the formation of a p-type nitride compound semiconductor layer has been realized, it is still impossible to increase the concentration of p-type carrier, and hence to lower the resistance of element as well as to lower the contact resistance of electrodes.
As explained above, the conventional process of manufacturing a p-type compound semiconductor layer is involved with a phenomenon which interferes with the aim of the p-type compound semiconductor. Further, it has been difficult according to the conventional method to form a p-type nitride compound semiconductor layer having a high carrier concentration.